Individuals within a population often display variation in sociosexual behaviors (e.g., pair-bonding; parental care), resulting in different patterns of reproductive behavior. The emerging model for the regulation of sociosexual behavior in mammalian species is complex, involving multiple neurogenic and environmental factors. Laboratory studies during the past two decades have provided a wealth of information on specific neurobiological mechanisms that regulate social behaviors associated with mating and reproduction. However, there remains a crucial need for studies within ecologically relevant contexts to validate these neurobiological regulatory mechanisms as biologically important in field populations. Data from laboratory studies demonstrate that estrogen, acting via estrogen receptor alpha (ER?), influences many aspects of sociosexual behavior in male mammals including affiliation, aggression, mating, and paternal care and suggest that variation in ER? expression is contributing to the mediation of these male behaviors in nature. The prairie vole (Microtus ochrogaster) is a popular laboratory model for studying neurobiological mechanisms of behavior involved in social monogamy. Nonetheless, male prairie voles in nature show considerable within and between population variation in social and genetic monogamy. Enhanced ER? expression in the medial amygdala (MeA) is associated with decreased prosocial behavior in male prairie voles under laboratory conditions. The study we are proposing will utilize naturally occurring variation in behavior and neuroanatomy between populations of prairie voles to test the effect of ER? expression on indices of male social and genetic monogamy among individuals released into 0.1 ha semi- natural enclosures. Our study will be the first to assess the role of ER? expression on sociosexual behavior in male prairie voles within a population-based and ecologically relevant framework to test the hypothesis that ER? expression is a biologically important mediator of male behavior in nature. Furthermore, we also plan to use RNAi to inhibit male ER? expression in the MeA to test for a direct causative effect of ER? expression in the MeA on male social and reproductive behavior, rather than just searching for a correlation as is often the case in field studies. We predict that males with highe ER? expression will have larger home ranges, show less home range overlap and social association with a single female, be less likely to be a resident at a female's nest, and sire offspring with more females than males with lower levels of ER? expression. Examining the consequences of variation in ER? expression in a milieu in which almost the complete repertoire of ecological and social influences on behavior can occur will provide information that cannot be obtained from laboratory experiments. This approach is the logical next step to explore whether individual differences in ER? expression affect male sociosexual behavior in nature and is essential for translating the results from lab findings using animal models into treating and understanding human social deficit disorders.